Secondary batteries using a hydrogen rechargeable negative electrode are known in the art. These batteries operate in a different manner than lead acid, nickel-cadmium or other battery systems. The rechargeable hydrogen storage electrochemical cell or battery utilizes a negative electrode that is capable of reversibly electrochemically storing hydrogen and usually employs a positive electrode of nickel hydroxide material, although other positive materials may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte, which may include a suitable separator membrane.
Upon application of an electrical current to the negative electrode, the negative electrode material (M) is charged by the absorption of hydrogen: EQU M+H.sub.2 O+e.sup.- M-H+OH.sup.- (Charging)
Upon discharge, the stored hydrogen is released to provide an electric current: EQU M-H+OH.sup.- M+H.sub.2 O+e.sup.- (Discharging)
The reactions are reversible.
The reactions that take place at the positive electrode are also reversible. For example, the reactions at a conventional nickel hydroxide positive electrode as utilized in a hydrogen rechargeable secondary cell or battery are: EQU Ni(OH).sub.2 +OH.sup.- NiOOH+H.sub.2 O+e.sup.-
(Charging) EQU NiOOH+H.sub.2 O+e.sup.- Ni(OH).sub.2 +OH.sup.-
(Discharging).
A battery utilizing an electrochemically rechargeable hydrogen storage negative electrode can offer important potential advantages over conventional secondary batteries. Hydrogen rechargeable negative electrodes offer significantly higher specific charge capacities than lead or cadmium negative electrodes. A higher energy density is possible with hydrogen storage batteries than with these conventional systems, making hydrogen storage batteries particularly suitable for many commercial applications.
Suitable active materials for the negative electrode are disclosed in U.S. Pat. No. 4,551,400 to Sapru et al, incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. The materials of Sapru et al have compositions of: ##EQU1##
Reference may be made to U.S. Pat. No. 4,551,400 for further descriptions of such materials and for methods of making them. Other suitable materials may also be used for the rechargeable hydrogen storage negative electrode.
One problem that has been encountered in rechargeable batteries is charge retention, also referred to as self discharge. Charge retention, or self discharge, describes the condition where an electrochemical cell loses stored energy over time through internal discharge mechanisms. While this problem is common to cells utilizing both cadmium negative electrodes and to hydrogen storage negative electrodes, it has heretofore been a greater problem with hydrogen storage negative electrodes. For example, while a typical nickel-cadmium cell loses about ten percent of its stored charge over a period of about one week at ambient temperatures, a cell utilizing a prior art metal hydride negative electrode, i.e., a prior art hydrogen storage negative electrode, may lose as much as thirty five percent of its stored charge over the same period. Furthermore, a cell utilizing a prior art metal hydride negative electrode tends to worsen in charge retention after cycling. This condition has been termed "aging". It is possible for a cell having a metal hydride hydrogen storage negative electrode typical of the prior art which initially loses charge at a rate of twenty five to thirty five percent per week, to lose charge at a rate greater than sixty percent per week after charge discharge cycling.
Self discharge in nickel-cadmium cells is generally attributable to two dominant mechanisms. One is related to oxygen evolution at the positive electrode, and the other is related to the presence of residual impurities, like nitrates, which act as a redox couple. These residual nitrate impurities are introduced into the positive electrode during fabrication.
The first mechanism relates to chemical instability of the positive (nickel hydroxide) electrode. Under normal circumstances, a nickel hydroxide positive electrode evolves oxygen at high states of charge and during overcharge. For sealed cells this condition is utilized to provide an overcharge reaction. The oxygen produced at the end of charge of the nickel hydroxide positive electrode recombines at the cadmium negative electrode to form cadmium oxides or hydroxides. This reaction can be chemical or electrochemical. The process can be sustained indefinitely. This oxygen recombination reaction can be thought of as a discharge mechanism to balance the charging mechanism during overcharge.
Ideally, when the overcharge current is removed, oxygen evolution should cease. However, practically it is possible for oxygen to be evolved for some time after the charge current is removed. The formation of high valence, non-stoichiometric nickel hydroxides/oxides and surface impurities are known to contribute to oxygen evolution. This evolved oxygen can also migrate to the negative electrode, and without additional charge current, discharge the cell. The degree of this reaction is small, but variable, usually attributed to about 3% to 5% capacity loss (of the overall 10 percent capacity loss) in one week. This reaction is also related to the state of charge of the nickel hydroxide; so as the cell is discharged by this reaction, and the possible electrode state of charge is reduced, the reaction becomes virtually negligible.
The second reaction mechanism generally associated with self discharge in nickel-cadmium cells is commonly referred to as the "nitrate shuttle". At various stages of positive nickel hydroxide electrode fabrication, it is possible for nitrate ions to be unintentionally incorporated into the positive electrode and carried into the cell. These residual nitrate ions, together with the reduced form (nitrite), are the basis of the redox shuttle mechanism. Nitrites diffuse through the electrolyte to be oxidized to nitrates at the positive electrode and then diffuse back through the electrolyte to be reduced to nitrites at the negative electrode. This nitrate-nitrite redox couple effectively reduces cell capacity during this process. It has been reported that very low concentrations of nitrate impurities, on the order of 200 parts per million, can be associated with generally observed self discharge rates in nickel-cadmium cells.
It is probable that these two mechanisms are also present in cells having hydrogen storage negative electrodes, although to possibly different degrees than that found in nickel-cadmium cells. As a practical matter, it is difficult to quantify individual degrees for each reaction in relation to the overall self discharge rate. Moreover, it is also likely that additional mechanisms for self discharge can exist in cells utilizing prior art metal hydride negative electrodes in place of cadmium negative electrodes.
Nickel hydroxide positive electrodes are also used in cells having hydrogen storage negative electrodes. One goal of positive electrode fabrication (to achieve high energy density cells) is to provide an electrode that is more heavily loaded in nickel hydroxide. It is possible that a nickel hydroxide positive electrode which is heavily loaded in nickel hydroxide for use with a hydrogen storage counter electrode may have a greater tendency towards oxygen evolution, which would increase the rate of self discharge in a cell.
The level of nitrate ion impurities incorporated in more heavily loaded positive electrodes is greater than that found in nickel-cadmium cells. The fabrication processes used for producing these positive electrodes are similar to those used for conventional nickel cadmium batteries. However, because of the high loading of the electrodes, it is possible for the high capacity nickel hydroxide positive electrodes to introduce higher concentrations of nitrate impurities. These higher levels of residual nitrate impurities, coupled with heavily loaded nickel hydroxide electrodes, are less efficiently removed during processing than is the case with prior art low capacity nickel hydroxide positive electrodes. Like the chemical stability of the positive electrode, it is difficult to quantify the degree of this specific self discharge mechanism in the presence of other self discharge mechanisms.
We have observed, for example, that some of hydrogen storage electrode materials of U.S. Pat. No. 4,551,400 to Sapru et al, while characterized by high capacity, high charge rates, high discharge rates under load, and high cycle life, are also highly susceptible to high rates of self discharge. The overall self discharge rate is probably influenced by both the chemical instability of the positive electrode and the nitrate ion redox mechanism.
The nitrate impurity appears to increase self discharge by a nitrate shuttle self discharge mechanism. This nitrate shuttle self discharge mechanism is specifically an oxidation/reduction, or redox, mechanism. The nitrate ion impurity incorporated in and introduced through the positive electrode has the electrochemical property of being able to exist as a nitrate ion (NO.sub.3.sup.-), or as a nitrite (NO.sub.2.sup.-) ion. The mechanism can be basically stated in the following manner. Nitrate ions are initially present in a cell as residual impurities from the positive electrode fabrication process. These residual nitrate ions are only present in small quantities, typically in the range of 200 parts per million. These nitrate ions diffuse through the alkaline electrolyte to the negative electrode, which can be a cadmium or metal hydride electrode. At the negative electrode the nitrate ion (NO.sub.3.sup.-) is electrochemically reduced to nitrite ion (NO.sub.2.sup.-). The reduction step at the negative electrode lowers the stored charge of the negative electrode. The reduced nitrite ion (N.sub.2.sup.-) then diffuses to the positive nickel hydroxide electrode where it is oxidized back to nitrate (NO.sub.3.sup.-). Thus, this process is essentially cyclical, and over time, can ultimately self discharge all of the stored energy in the cell.
The nitrate shuttle self discharge process is affected by several factors. The concentration of nitrate impurities is important. If variations occur in the fabrication of the nickel hydroxide electrode, it is possible for the level (concentration) of residual nitrates to be very high. Moreover, because the nitrate shuttle is a diffusion process, the physical geometry of the cell is also important. The electrolyte level within the cell and the distance between the positive and negative electrodes are important, as well as separator qualities like porosity, pore size, and thickness. For a cell utilizing a metal hydride, hydrogen storage negative electrode, the construction may be of a jelly roll configuration, with a starved electrolyte, and a 0.008" distance between the two electrodes, and separated by a nonwoven nylon separator. These conditions are essentially the same as in a conventional nickel-cadmium cell.
The nitrate shuttle self discharge process is also affected by kinetic and thermodynamic considerations. High temperatures will increase the diffusion rate of the nitrate ions and the reaction rate of the redox reactions. The shuttle process is also affected by thermodynamic considerations, or state of charge. This means that the reaction is faster when the electrodes are at higher states of charge. Finally, it must be noted that the redox mechanism is actually an oxidation reaction at the positive electrode and a reduction reaction at the negative electrode. Consequently, the shuttle reaction can be affected by the surface characteristics of both electrodes; namely surface area, surface morphology, and catalytic nature.
Some hydrogen storage negative electrodes, e.g., hydrogen storage electrodes of the V-Ti-Zr-Ni type, may be manufactured to have very high surface areas. These large surface areas provide enhanced electrochemical properties. However, the high rates of self discharge for the V-Ti-Zr-Ni family materials may be at least partly due to the same higher surface area. This is because the higher surface area that provides enhanced electrochemical properties also provides greater reaction surface for the nitrate to nitrite reduction reaction.
For the materials of type V-Ti-Zr-Ni, the self discharge rate may also be high due to the type of surface. The nitrate to nitrite reduction mechanism appears to be accelerated in the V-Ti-Zr-Ni materials. This could be due to a higher concentration of conductive components at the metal/electrolyte interface. The higher concentration of metallic components, specifically nickel, in the alkaline electrolyte medium, may be catalytic to the nitrate reduction mechanism.
It is also possible that the type of surface present at the metal hydride negative electrode may be more favorable to other redox mechanisms than the surface of cadmium electrodes. In addition to oxygen produced at the positive electrode during overcharge, oxygen can also be dissolved in the electrolyte. The oxygen may diffuse through the thin layer of electrolyte present in starved cells to the negative electrode, where it may be electrochemically reduced to form peroxide ions or hydroxyl ions according to: EQU O.sub.2 +2e.sup.- +H.sub.2 O HO.sub.2.sup.31 +OH.sup.-
or EQU O.sub.2 +4e.sup.- +2H.sub.2 O 4OH.sup.-
The nature of the reaction path and the rate of reaction are highly dependent on the catalytic activity of the reaction surface. It is possible that the surface present with a metal hydride electrode has a suitable catalytic surface for these reactions. Since HO.sub.2.sup.- ions may diffuse to the positive electrode to be oxidized, and then repeat the reduction process at the negative electrode, the rate of self discharge may be affected.
Another impurity ion redox self discharge mechanism appears to be associated with the vanadium component introduced into a cell utilizing metal hydride negative electrodes of the type V-Ti-Zr-Ni, by the vanadium-containing metal hydride negative electrode disclosed in U.S. Pat. No. 4,551,400.
It is likely that vanadium is present in the alkaline electrolyte predominantly in its +5 oxidation state, and acts in a redox mechanism which contributes to self discharge in the cell. Vanadium is easily oxidized in the highly alkaline medium used in commercially practical electrochemical cells (30% potassium hydroxide in water). Once the vanadium is oxidized at the metal/electrolyte surface, the vanadium pentoxide oxidation product is readily soluble in the electrolyte.
A vanadium oxide shuttle mechanism occurs between the +4 and +5 oxidation states of vanadium. Both oxidation states are stable in aqueous alkaline media at the potentials present in the rechargeable cell utilizing a metal hydride negative electrode and a nickel hydroxide positive electrode. In the proposed vanadium shuttle, the V.sup.+5 component diffuses to the negative electrode where it is reduced to the V.sup.+4 oxidation state. Similarly, the V.sup.+4 component diffuses to the positive nickel hydroxide electrode where it is oxidized back to the V.sup.+5 oxidation state. Many of the factors of importance to the nitrate-nitrite redox couple mechanism are believed to be important for governing the rate of the vanadium (+4)-vanadium (+5) redox couple mechanism, e.g., concentration of vanadium oxides, physical aspects of cell construction, temperature, state of charge, and reaction surfaces of both electrodes.
The addition of vanadium oxide to the alkaline electrolyte has been demonstrated to increase the self discharge rate of commercially available nickel-cadmium cells. A standard nickel-cadmium cell having a self discharge rate of 10% loss in one week, was measured to have increased in self discharge rate upon the deliberate addition of vanadium pentoxide to the electrolyte. This experiment is shown in the examples.
A strong motivation for using the V-Ti-Zr-Ni family of electrochemical hydrogen storage alloys is the inherently higher discharge rate capability under load compared to materials of the V-Ti-Cr-Ni type. An important physical quality in this regard is substantially higher surface areas for the V-Ti-Zr-Ni materials. Measured in surface roughness (total surface area divided by geometric surface area), the V-Ti-Zr-Ni materials can have roughnesses of about 10,000, compared to about 3000 for some materials such as those of the V-Ti-Cr-Ni type. The very high surface area plays an important role in the inherently high rate capability of these materials. However, it is possible that the same increase in electrode surface area which contributes to inherently higher discharge rate capability under load for these materials may also contribute to higher rate of self discharge. For both the nitrate redox shuttle and the vanadium redox shuttle, the reaction rate at the negative electrode appears to have properties that are consistent with a surface catalyzed reduction. The high surface area of the V-Ti-Zr-Ni negative electrode hydrogen storage materials may promote the reduction step of the redox reaction, and the concomitant overall self discharge rate.
The metal/electrolyte interface also has a characteristic roughness. The characteristic surface roughness for a given negative electrode electrochemical hydrogen storage material is important because of the interaction of the physical and chemical properties of the host metals in an alkaline environment. The oxidation and corrosion characteristics of the host elements of the electrochemical hydrogen storage material are believed to be important in determining the oxidation and corrosion characteristics of the hydrogen storage material. Since all of the elements are present throughout the metal, they are also represented at the surfaces and at cracks which form the metal/electrolyte interface. For example, while vanadium corrodes easily, forming oxides which have a high solubility in the alkaline electrolyte, the oxides of titanium and zirconium are quite insoluble. For this reason titanium and zirconium do not corrode. Nickel is stable in its metallic state, by forming a thin passive oxide at the metal/electrolyte interface.
However, we have observed a high degree of vanadium corrosion in alkaline aqueous media from surfaces of hydrogen storage negative electrodes fabricated of vanadium, titanium, zirconium, and nickel. The titanium, zirconium, and nickel components of the hydrogen storage alloy do not seem to provide any degree of passive protection to the vanadium. Thus, titanium oxide, zirconium oxide, and metallic nickel apparently do not inhibit vanadium corrosion substantially. In fact, it has been observed that these reaction products are found as particles or colloidal suspensions during vanadium oxide corrosion.
On a microscopic scale, there appears to be little evidence of a self limiting corrosion process at the hydrogen storage electrode - electrolyte interface. Thus, with time, the surface increases its roughness. That is, a given unit surface area becomes rougher due to the corrosive properties of its constituent oxides, and the leaching and dissolution of these oxides as solids increases the overall surface area. Additional surface area, whether created through crack propagation or corrosion and/or erosion, promotes the reduction step of ion shuttle redox mechanisms, thus increasing self discharge.
In addition to the physical nature of the roughened surface, it has been observed that the V-Ti-Zr-Ni materials reach a steady state surface condition. This steady state surface condition is characterized by a relatively high concentration of nickel. The surface nickel is in the metallic state. These observations are consistent with a relatively high rate of removal of the oxides of titanium and zirconium from the surface and a much lower rate of nickel removal during vanadium corrosion. The resultant surface seems to have a higher concentration of nickel than would be expected from the bulk composition of the negative hydrogen storage electrode. Nickel in the metallic state is electrically conductive and catalytic, imparting these properties to the surface. As a result, the surface of the negative hydrogen storage electrode is more catalytic and conductive than if the surface contained a higher concentration of insulating oxides.
The surface, having a conductive and catalytic component, e.g., the metallic nickel, appears to assist the reduction step of the redox ion shuttle mechanism by catalyzing the reduction reaction.
Thus, the four component system which gives rise to high charge capacity, a high charge rate, and a high discharge rate under load, also gives rise to a high self discharge rate and a high aging, or degradation, effect.
Aging effect refers to the condition of the cell having a metal hydride negative electrode where the self discharge rate for a given cell increases after electrochemical cycling. For example, a prior art cell with a self discharge rate of about 25% loss in capacity per week at 25 degrees Celsius may increase to over 50% loss in 1 week after only 50 electrochemical cycles. No such effect is present in conventional nickel-cadmium cells.
It is believed that many of the same conditions which contribute to the initial self discharge rate are also responsible for the aging effect, or degradation of charge retention after electrochemical cycling.